Debt (Details 3) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Senior notes | Carrying Value
Notes and other indebtedness
Amount outstanding	$ 370.0	$ 370.0
Senior notes | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	390.6	387.0
Senior Notes - Series A | Carrying Value
Notes and other indebtedness
Amount outstanding	40.0	40.0
Senior Notes - Series A | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	40.2	40.6
Senior Notes - Series B | Carrying Value
Notes and other indebtedness
Amount outstanding	105.0	105.0
Senior Notes - Series B | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	109.9	110.0
Senior Notes - Series C | Carrying Value
Notes and other indebtedness
Amount outstanding	65.0	65.0
Senior Notes - Series C | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	68.7	68.3
Senior Notes - Series D | Carrying Value
Notes and other indebtedness
Amount outstanding	160.0	160.0
Senior Notes - Series D | Estimated Fair Value
Notes and other indebtedness
Amount outstanding	$ 171.8	$ 168.1